One of the major problems associated with using low rank coals such as those found in several of the western states of the United States is their high moisture content, usually ranging from about 15 to 50 weight percent. When such coal is shipped to the consumer, the high moisture content involves a large weight penalty and increased shipping expense. Also, upon firing such coal, considerable heat is required to vaporize this high moisture content, which reduces process efficiency. However, if the coal is dried to a very low moisture content before shipment, it experiences significant reabsorption of moisture and consequential heating as soon as subject to the air. This makes the coal subject to spontaneous ignition during shipment and/or subsequent storage and has resulted in serious fires.
The desirability of drying such high moisture coals and passivating them so as to substantially prevent the reabsorption of moisture has been recognized. For example, U.S. Pat. No. 1,905,513 to Stuart describes a method for filming (coating) coal with a preserving hydrocarbon film which is impervious to both air and water so as to help prevent dusting and oxidation of the coal before burning. U.S. Pat. No. 1,960,917 to Nagelvoort describes a method for removing excess moisture from wetted coal by spraying it with a dilute oil emulsion to facilitate the drainage of excess water from the coal. Also, U.S. Pat. No. 2,197,792 to Erickson describes apparatus for spraying of coal with oil or wax to prevent dusting, while Wattles -- U.S. Pat. No. 2,204,781 -- describes coating exposed surfaces of coal piles with a protective weather-excluding coating material. Furthermore, Lykken in U.S. Pat. No. 2,610,115 and U.S. Pat. No. 2,811,427 describes a method for dehydrating lignite by mixing it with 3-10 weight percent mineral hydrocarbon at normal temperature and then heating the mixture to about 300.degree. F in a rotating kiln to remove moisture and leave the lignite particles coated with the hydrocarbon material.
While the problem of spontaneous combustion of low rank coals is well recognized and over a period of more than fifty years many capable scientists have offered solutions, usually based on small laboratory tests, there is still no practical process for accomplishing passivation on both a large scale and at an economically low cost.